The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to a method and system for maintaining the homogeneity of a magnetic field within the imaging system over a range of passive shim temperatures.
Magnetic Resonance Imaging (MRI) often relies on a homogeneous magnetic field in order for improvement of the resultant medical images. In order to develop the desired homogeneous magnetic field, it is common for ferromagnetic elements used as passive shims to be placed within the bore of the imaging magnet. Often, these passive shims are supplemented through the use of electrically powered field correction coils, frequently referred to as resistive shims in order to increase their ability to affect the magnetic field. The position of the passive shims must be carefully chosen such that their effect on the magnetic field results in a more homogeneous field.
Although the passive shims have been successful in improving magnetic field homogeneity, it is known that many passive shims, such as iron pieces, may be effected by temperature. As the imaging system is operated, the passive shims may experience changes in temperature. For many passive shims, this change in temperature may effect the magnetization of the shims and thereby affect the magnetic field. This not only changes the strength of the magnetic field within the bore, but the field may also become less homogenous as the temperature of the passive shims varies. This negative impact on the homogeneity of the magnetic field can result in a loss of image quality.
One approach to eliminating the impact of passive shims, such as iron, on the magnetic field has been to minimize the amount of materials such as iron utilized. This runs counter to the positive benefits provided by the iron or similar shims when thermally stable. Other approaches have turned to alternative shimming solutions such as superconductive coils. These alternative solutions, however, often are associated with expensive implementation costs that work against their suitability or practicality.
It would, therefore, be highly desirable to have a magnetic resonance imaging assembly with improved control over magnetic field homogeneity. It would additionally be highly desirable to have such improved control over magnetic field homogeneity without the costs associated with many known alternative shimming solutions.
An imaging system is provided comprising an imaging magnet having a magnet bore. A plurality of passive shims is positioned within the magnet bore. A plurality of resistive shims is also positioned within the magnet bore. A plurality of thermometers are coupled thermally to a select number of the plurality of passive shims and read a passive shim temperature. A controller is in communication with the plurality of thermometers and the plurality of resistive shims. The controller includes logic adapted to adjust control currents sent to each of the plurality of resistive shims in response to the passive shim temperatures received from each of the thermometers such that magnetic homogeneity is maintained. Other features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.